What does a pan balance tell us about the weight order of the cubes?
1. Ask the question
Set up a pan balance where all students can see it.
- What do you suppose this is?
- What could it be used for?
- How do you think it might work?
Note: Students may suggest that the pan balance is used to “weigh things.” Return to that thought later, once students have seen that a pan balance can also compare the weights of objects without actually weighing them.
Check students’ understanding by having them predict what they will observe if the objects in the two pans weigh the same. Invite a student to put a steel cube in each pan. Did students predict correctly?
Next ask students to predict what they will observe if the object in this pan is heavier than the object in that pan. Invite a student to place two different cubes in the pans. Can the students explain what happens? Why do they think what they think?
Letter from the Engineer
When you were using your senses to compare the weights of cubes, sometimes that worked well, but sometimes it was very difficult — using just your hand and your senses — to decide which cube was heavier. When the weight difference between two objects is very small, our senses may not notice the difference; they are not reliable at telling us which object is heavier. That’s the kind of a problem that engineers can solve. They have designed a tool to compare the weights of objects that is more effective than our senses.
Explore students’ readiness to take the next step. Calling attention to the class table they created last session.
- How can this pan balance help us check our predicted order of the cubes?
Select three cubes and have a willing student demonstrate how the pan balance can check the orderings that they arrived at by felt weight. Check to see if students understand how to use the balance in this way.
Finally, share the new investigation question:
